1. Field of the Invention
The present invention relates to a wireless communication apparatus and a wireless communication method that carry out MIMO (Multi Input Multi Output) communication in which multiple logical channels are formed by using spatial multiplexing. In particular, the invention relates to a wireless communication apparatus and a wireless communication method that further enhance communication capacity by carrying out spatial multiplexing transmission in which some weighting (beam forming) is performed at a transmitter.
More specifically, the invention relates to a wireless communication apparatus and a wireless communication method that perform spatial multiplexing transmission operation with high transmission efficiency by using increased communication capacity obtained by performing at a transmitter optimal allocation of transmission power to transmit streams. In particular, the invention relates to a wireless communication apparatus and a wireless communication method that allows a receiver to spatially demultiplex, with high performance, a spatially multiplexed signal composed of multiple transmit streams to which power is allocated.
2. Description of the Related Art
Wireless networks draw attention as systems that free users from cable wiring in traditional wire communication schemes. Canonical standards for the wireless networks include IEEE (the Institute of Electrical and Electronics Engineers) 802.11.
For example, in IEEE802.11a/g, an OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme which is one of the multicarrier schemes is adopted as a canonical standard for wireless LANs. In the OFDM modulation scheme, the frequencies of subcarriers are set such that the subcarriers are orthogonal to each other in a symbol section. That the subcarriers are orthogonal to each other signifies that the peak point of the spectrum of an arbitrary subcarrier always matches the zero point of the spectra of other subcarriers. According to the OFDM modulation scheme, transmit data streams are allocated to multiple carriers having different frequencies and transmitted; therefore, the bandwidth of each carrier becomes narrow, thus providing high frequency utilization efficiency and high resistance to frequency-selective fading interference.
An OFDM transmitter converts information from serial to parallel form for each symbol period which is slower than an information transmission rate, allocates a plurality of converted data to subcarriers, modulates the amplitude and phase of each subcarrier, transforms the modulated signals in to signals on the time axis while maintaining the orthogonality of each subcarrier on the frequency axis by performing an inverse FFT on the multiple subcarriers, and transmits the transformed signals. Further, as the inverse operations, an OFDM receiver transforms signals on the time axis to signals on the frequency axis by performing an FFT, demodulates each subcarrier in accordance with each modulation scheme, and converts the demodulated signals from parallel to serial form to reproduce the information of an original serial signal.
The IEEE802.11a standard supports a modulation scheme achieving a communication speed of up to 54 Mbps. However, there is a need for additional standards that can realize a higher bit rate of communication speed.
The MIMO communication attracts attention as one technology for realizing higher-speed wireless communication. The MIMO communication scheme achieves spatially multiplexed transmission channels (hereinafter referred to as “MIMO channels”) in a MIMO system where both transmitter and receiver have multiple antenna elements. The transmitter allocates transmit data streams to the multiple antennas and transmits them through MIMO channels. At the other end, the receiver can extract receive data without crosstalk through signal processing on the spatially multiplexed signals received by its multiple antennas (e.g., see patent document 1). For example, in the standardization work of IEEE802.11n, there are continuing discussions centering on methods for realizing high-speed wireless communication by combining OFDM adopted in IEEE802.11a/g and the above-described MIMO communication scheme.
The MIMO communication scheme can achieve enhancement in communication capacity and communication speed in accordance with the number of antennas without increasing the frequency band. Further, the MIMO communication scheme uses the spatial multiplexing, so that it exhibits high frequency utilization efficiency. The MIMO communication scheme exploits channel characteristics, unlike simple adaptive antenna arrays at the transmitter and the receiver.
In general, the channel model is configured of a radio environment (transfer function) around the transmitter, a channel space structure (transfer function), and a radio environment (transfer function) around the receiver. Multiplexed signals transmitted from the antennas involve crosstalk. However, the receiver can correctly process the multiplexed signals without crosstalk through reception processing in accordance with channel characteristics.
FIG. 9 conceptually shows a MIMO communication system. A MIMO transmitter is provided with two antennas, namely, transmit antenna 1 and transmit antenna 2. At the other end, the receiver is also provided with two antennas, namely, receive antenna 1 and receive antenna 2. In FIG. 9, propagation path a denotes the propagation path between transmit antenna 1 and receive antenna 1. Propagation path b denotes the propagation path between transmit antenna 2 and receive antenna 1. Propagation path c denotes the propagation path between transmit antenna 1 and receive antenna 2. Propagation path d denotes the propagation path between transmit antenna 2 and receive antenna 2. Further, the transmitter allocates transmit data series X1 and X2 to transmit antennas 1 and 2, respectively. The receiver receives receive data series Y1 and Y2 at receive antennas 1 and 2, respectively. In this case, the conditions of the propagation paths are expressed in the following equation (1).
                              (                                                                      Y                  ⁢                                                                          ⁢                  1                                                                                                      Y                  ⁢                                                                          ⁢                  2                                                              )                =                              (                                                            a                                                  b                                                                              c                                                  d                                                      )                    ⁢                      (                                                                                X                    ⁢                                                                                  ⁢                    1                                                                                                                    X                    ⁢                                                                                  ⁢                    2                                                                        )                                              (        1        )            
When a channel matrix H in this case is defined as the following equation (2), the inverse matrix H−1 of the channel matrix H as an antenna receive weight matrix W is expressed as the following equation (3).
                    H        =                  (                                                    a                                            b                                                                    c                                            d                                              )                                    (        2        )                                          H                      -            1                          =                              (                                                            a                                                  b                                                                              c                                                  d                                                      )                                -            1                                              (        3        )            
Therefore, by multiplying receive signal series Y1 and Y2 by the inverse matrix H−1 of the channel matrix H as shown in the following equation (4), receive signal series X1 and X2 are expressed in the following equation (5).
                              (                                                                      X                  ⁢                                                                          ⁢                  1                                                                                                      X                  ⁢                                                                          ⁢                  2                                                              )                =                                            (                                                                    a                                                        b                                                                                        c                                                        d                                                              )                                      -              1                                ⁢                      (                                                                                Y                    ⁢                                                                                  ⁢                    1                                                                                                                    Y                    ⁢                                                                                  ⁢                    2                                                                        )                                              (        4        )            
Two transmit antennas and two receive antennas are shown in FIG. 9. However, as long as the number of antennas is two or more, a MIMO communication system can be constructed in the same way. The transmitter space-time encodes multiple transmit data streams, multiplexes the encoded data, allocates the multiplexed signals to M transmit antennas, and transmits them onto MIMO channels. The receiver receives the multiplexed transmit signals by N receive antennas through the MIMO channels and space-time decodes the received transmit signals to obtain receive data. The number of formed MIMO streams ideally matches the number of transmit antennas M or the number of receive antennas N, whichever is smaller, min[M, n].
In order to spatially demultiplex the spatially multiplexed receive signals y into the stream signals x as descried above, the MIMO receiver needs to acquire the channel matrix H in some way and obtain the receive weight matrix W from the channel matrix H in accordance with a predetermined algorithm.
For example, the transmitter transmits training signals composed of known signal series, and the receiver can acquire the channel matrix H using the training signals.
Further, as a relatively simple algorithm for obtaining the receive weight matrix W from the channel matrix H, there is known Zero Force (e.g., see non-patent document 1) and MMSE (Minimum Mean Square Error) (e.g., see non-patent document 2). Zero Force is a method based on the logic of completely eliminating crosstalk. On the other hand, MMSE is a method based on the logic of maximizing the ratio of signal power to square error (sum of crosstalk power and noise power). In MMSE, the receive weight matrix W (inverse matrix of the channel matrix) is obtained by generating crosstalk intentionally, under the concept of noise power of the receiver. It is known that MMSE is superior to Zero Force in a high-noise environment.
As described above, by disposing multiple transmit/receive antennas, the MIMO communication system can enhance communication capacity without increasing the frequency band. The communication capacity of MIMO transmission can be further enhanced by carrying out spatial multiplexing transmission in which some weighting (beam forming) is performed at the transmitter.
The problem of how to allocate transmission power to maximize the overall communication capacity can be solved by, for example, a water filling principal (e.g., see non-patent document 3). The water filling principal refers to a principal that sets each transmission power to a value obtained by subtraction from the amount proportional to attenuation of the channel. According to the water filling principal, the transmitter performs power allocation by which higher transmission power is allocated to a channel in good condition and lower power is allocated to a channel in poor condition, thus maximizing the communication capacity of the MIMO communication system.
Consideration will be given to the case where the MMSE is employed in the reception process in MIMO communication. In the MMSE process, noise power is added to the diagonal elements of the channel matrix created from training signals, thereby to cancel interference and noise adjusting the balance between the interference and the noise and to acquire desired signal components. The MMSE is superior under a high-noise environment.
It should be noted that acquiring a channel matrix from training signals having the same power and demultiplexing signals in accordance with the MMSE process is predicated on the allocation of the same power to each stream.
On the other hand, the communication capacity is maximized, for example, by allocating power proportional to the eigenvalue of a channel at the time of carrying out MIMO communication in which some weighting transmission (beam forming) is performed at the transmitter, as described above.
However, there arises a problem when the receiver receives streams subjected to power allocation at the transmitter in accordance with MMSE. This is because the MMSE process is predicated on the allocation of the same power to each stream. In other words, if the receiver performs spatial demultiplexing in accordance with the MMSE process without consideration of per-transmit-stream power allocation values, an accurate antenna weight matrix cannot be obtained. This is not the original MMSE process, thereby causing degradation in characteristics so that the communication capacity is not maximized.
Further, as a method by which the receiver acquires a power allocation value allocated to each stream at the transmitter, there is a possible method by which the transmitter transmits information on the power allocation values. However, since this method requires the provision of an extra header field that does not contribute to information transmission, it is not desirable from the viewpoint of communication capacity.
[Patent document 1] Japanese Patent Application Laid-Open No. 2002-44051
[Non-patent document 1] A. Benjebbour, H. Murata and S. Yoshida, “Performance of iterative successive detection algorithm for space-time transmission”, Proc. IEEE VTC Spring, vol. 2, pp. 1287-1291, Rhodes, Greece, May 2001.
[Non-patent document 2] A. Benjebbour, H. Murata and S. Yoshida, “Performance comparison of ordered successive receivers for space-time transmission”, Proc. IEEE VTC Fall, vol. 4, pp. 2053-2057, Atlantic City, USA, September 2001.
[Non-patent document 3] G. J. Foschini and M. J. Gans, “On limits of wireless communications in a fading environment when using multiple antennas” (Wireless Personal Communications, vol. 6, no. 3, pp. 311-335, March 1998)